Abstract: A safety valve for a second stage regulator of a self-contained underwater breathing apparatus including a longitudinally slideable valve stem for selectively interrupting flow between a source of pressurized gas and a second stage regulator. The valve is mounted in line intermediate the first stage regulator and the second stage regulator of the apparatus. The valve has a closed position wherein fluid flow between the first and second stage regulators is cut off and an open position wherein such flow is permitted. Thus, prior to a dive the valve is in its closed position so as to minimize the likelihood of gas leakage from the second stage regulator. When the second stage regulator is to be used, the valve is slid to its open position and pressurized gas is free to flow to the second stage regulator.